Optically active α-hydroxycarboxylic acids are used, for example, as additives to feeds, or in the production of pharmaceutical active compounds, vitamins and liquid crystals.
These optically active α-hydroxycarboxylic acids may, in addition, be advantageously converted, for example according to Effenberger et al., Angew. Chem. 95 (1983) No. 1, page 50, into N-substituted optically active α-amino acids which are otherwise prepared only with great difficulty.
Chiral α-hydroxycarboxylic acids are nowadays accessible chemically, by fermentation, or enzymatically.
The literature accordingly discloses a number of various methods for synthesis of chiral α-hydroxy-carboxylic acids.
For instance, racemic cyanohydrins, with addition of suitable microorganisms, can be hydrolyzed to give the desired chiral α-hydroxycarboxylic acids.
Production of chiral α-hydroxycarboxylic acids, especially the production of optically active lactic acid or mandelic acid, from racemic cyanohydrins using various microorganisms of the genera Alicaligenes, Pseudomonas, Acinetobacter, Rhodococcus, Candida etc. is described, for example, in EP 0 449 684, EP 0 527 553, EP 0 610 048, etc.
From this prior art, it is also known that when a racemic cyanohydrin is enzymatically hydrolyzed to the conjugate α-hydroxycarboxylic acid in the presence of a nitrilase, the problem occurs that the enzyme is inactivated within a short time and thus the desired α-hydroxycarboxylic acid is usually abtained only in low yields and concentrations. This also applies to the use of nitrile hydratases, which convert the cyanohydrin to the conjugate α-hydroxyamide. The hydroxyamides can then in turn be converted to the conjugate α-hydroxycarboxylic acids.
It is also known, for example from Angew. Chem. 1994, 106, page 1615f., that optically active cyanohydrins may be hydrolyzed by concentrated hydrochloric acid, without racemization, to give the conjugate chiral α-hydroxycarboxylic acids. The optical purity of the chiral α-hydroxycarboxylic acids thus produced corresponds here to the optical purity of the chiral cyanohydrin used, even if this is obtained in situ by enzyme-catalyzed addition of a cyanide group to a conjugate aldehyde or a ketone and is further processed without isolation or purification.
It is disadvantageous with this reaction that sensitive substrates are decomposed, and the occurrence of corrosion.